1. The Field of the Invention
This invention relates generally to measuring analytes in samples based on an electromagnetic spectrum that is characteristic of the analyte and, more specifically, doing so by use of multiplexed holograms and polarization manipulation.
2. Background and Relevant Art
Many attempts have been made to create appropriate apparatus for the non-invasive measurement of significant substances within biological organisms. The importance of such measurement capability arises not only from the need to observe biochemical reactions in such organisms without disturbance to the system but also in order to help control chronic diseases such as diabetes, where it is highly desirable to measure the patients blood glucose levels much more frequently than is practical, when puncturing the skin is required.
Spectroscopy has been proposed to make such measurements. An important consideration in the design of spectrometers is the field of view of the apparatus for a given resolution. In particular, it is often advantageous to use a large detector so as to image a larger volume of the sample. However, it is generally undesirable to increase the diameter of the collecting optics in proportion. Therefore, it is usually desirable to have as large a field of view as possible as a fraction of the diameter of the collection optics.
U.S. Pat. No. 5,768,040 presents a spectrometer arrangement which as a composite has a wide field of view. The apparatus is composed of multiple spectrometers, each of which is based on spherical convex holographic diffraction gratings and associated reflection optics. The cost of the whole apparatus is comprised of a multiplicity of these spectrometers and is prohibitive for many applications. In addition, the size of such an apparatus would preclude its use where the available space is limited. A solution where effectively multiple spectrometers can be multiplexed in the same available space and the cost does not scale strongly with the field of view would be preferable.
Multiple reflection holograms, multiplexed in a single recording medium, can advantageously be used to reflect each useful spectral line of the analyte to either a single detector or to multiple detectors. However, as the limiting aperture of the optical system becomes an appreciable fraction of the diameter of the collecting lenses or mirrors, it is unavoidable that the holograms will be illuminated with light distributed over a range of angles rather than by perfectly collimated light. The holograms, however, have limited acceptance angle over which they have adequate diffraction efficiency. Hence, the amount of light to be collected will typically scale positively with the acceptance angle of the holograms. The range of angles can be reduced at the expense of using larger diameter optics, but with detrimental effects on cost and size. A large field of view for the holograms can obviate the need for larger optics or, for a given optics size, can increase the amount of light which is collected. It can be readily shown that for plane holograms, the field of view is maximized for near normal incidence illumination with respect to the fringes that comprise the hologram. However, it can be exceptionally awkward to illuminate reflection holograms at near 90° with respect to the surface, for the reflected light will then return substantially along the same path as the illuminating light and the two beams may be difficult to separate.
In addition, it is has been found experimentally that certain scattering processes may produce scattered light which is predominantly polarized. The polarization may be well-preserved, even after passing through a turbid medium (such as human tissue) if the scattered light has not been deflected substantially from its original direction. Inelastic processes such as Raman scattering can provide scattered spectra which contain characteristic spectral signatures of various analytes of interest. If the Raman scattered radiation is collected from a depth within the turbid sample, which is not excessive, it has been found that the scattered radiation is highly polarized.
Fluorescence arising from the excitation of substances which absorb at the exciting laser wavelength is often the dominant source of noise. The fluorescence is generally found to be only weakly polarized, but may be orders of magnitude greater in amplitude than the Raman signals. If a quantitative estimate of the concentration of an analyte is desired, it is desirable to devise an accurate method of subtracting the fluorescence signal from the Raman signal. In addition, it is desirable to preferentially attenuate the fluorescence with respect to the Raman signal in order to improve the signal to noise ratio. It is therefore desirable to find a method whereby the preferential polarization of scattered signals which occurs in some favorable circumstances can be fully exploited to improve signal to noise ratio, and accurately extract the interfering unpolarized radiation.
Approaches to spectroscopy that use dispersive elements such as diffraction gratings suffer from drawbacks. For example, many of these approaches cannot multiplex two or more spectral lines of an analyte onto a single detector for purposes of increasing the signal. Techniques which use cascaded dichroic transmission filters have the same deficiency. Also, neither technique segregates the two polarizations for purposes of subsequent subtraction. The light which is polarized orthogonal to that of the desired signal should be spectrally filtered using filters with substantially the same spectral characteristics that are applied to the first polarization, for otherwise the subtraction of the two signals cannot accurately extract the noise.
Finally, it is frequently desirable to observe the spectral lines of one or more additional analytes in order to accurately establish the absolute concentration of the first analyte. This is particularly true when there are substances present with confounding optical spectra that can overlap the spectrum of the analyte at one or more wavelengths. Each confounding substance, however, may have one or more unique lines different from that of the analyte. Hence, observation of these other lines can be used to extract the contribution of the confounding substances. In a more general approach, a regression algorithm can be constructed to extract the contribution of multiple confounding substances.
Therefore it is desirable to alternatively view additional spectral lines distinct from that of the first analyte. Alternatively, it is often very useful to observe spectral lines of the solvent in which the analyte is dissolved to determine the quantity of the solvent in the scattering volume. Using the volume of solvent, the absolute concentration of the analyte can be determined. Generally observation of many spectral lines has required the implementation of a spectrograph with a linear array of detectors to observe the whole spectrum. This can be expensive when a large detector array is required. Furthermore, the dark current noise of multiple detectors is additive. Hence, the large array may have inferior signal to noise as compared with a smaller array or a single detector.
Thus, there is a need for improved spectroscopic approaches, for example as may be used to detect analytes based on their spectral lines.